In many applications of electrical machines comprising an inverter, current measuring sensors are required in the supply lines, in order to be capable of correctly detecting the current operating condition of the machine. For example, when controlling an asynchronous machine connected to an inverter, current measuring sensors are used in the individual supply lines, which make the measured current values available to a control device. It is thus possible to draw conclusions about the current operating condition of the machine to be controlled and to react accordingly to changes during the operation of the asynchronous machine. If three supply lines to the asynchronous machine are used, three current measuring sensors are required. It is known that by applying the Kirchhoff's junction rule one of the current measuring sensors may be saved without having to accept a loss of information about the currents in the supply lines, thus not only being capable of eliminating possible sources of errors but also of reducing costs.
If several electrical machines are operated in parallel on a common inverter, they may partially be exposed to different loads and therefore have deviating operating conditions. For a specific control it is therefore essential to determine or at least estimate the deviations between the individual machines.
The power supply of several drives with a common inverter is also referred to as “group drive” or “multi motor” in some citations. This concept is used, among other things, in tramways or trains and is constantly being further developed.
For example, the US 2009/0309529 A1 shows a system for operating several parallel (linear) motors, each of which has 3 supply lines. Current measuring sensors are arranged in each two of the supply lines to be able to determine the operating conditions of the individual motors and protect them from overload. For the actual control of the motors, however, other current measuring sensors are provided, which measure the total currents in the supply lines prior to the distribution to the individual motors.
The US 2006/0012322 A1 discloses a drive device for operating several parallel asynchronous machines (ASM) on an inverter. Current measuring sensors are provided in the common supply lines of the ASM, whose measured values can be used to determine a deviation measure for detecting load differences between the individual ASM. In addition, corresponding control measures are also proposed on the basis of this deviation measure.
The U.S. Pat. No. 4,298,831 also shows a device for the control of several parallel ASM, whereby current sensors are arranged in each one of the individual supply lines of the ASM.
The patent specification JP 2002112404A discloses several variants for controlling parallel ASM. An embodiment relates to the arrangement of current sensors in the same phases of several ASM, whereby different loads of the machines can be determined by comparing the measured current values.
The JP 2011 072062 A discloses an interconnection of at least two permanent-magnet-excited synchronous machines in BLDC mode, which are operated on a common inverter. The current sum of two supply currents of the first synchronous machine and of a supply line current of the second synchronous machine is determined with the aid of a total current transformer, in order to be able to detect any failures of the synchronous machines during operation with the aid of the sign of the current sum. The disadvantage of JP 2011 072062 A, however, is that no information is available on the operating conditions of the individual synchronous machines, so that no current space phasor can be calculated and no vector control or field-oriented control can be used. Furthermore, in the JP 2011 072062 A, due to the lack of information on the currents in the individual supply lines, no reference value can be determined, with which the total current ascertained could be related, in order to determine a relative deviation between the individual machines.
In addition, from US 2007/273310 A1 a parallel connection of two electrical machines on a common inverter is known, wherein current measuring sensors are inserted in at least two supply lines of each machine. It is a disadvantage that the number of current measuring sensors increases with the number of machines used.
A disadvantage of the conventional art is the high number of current measuring sensors required, which are arranged in three or two motor supply lines in a frequently found variant. Thus, the number of current measuring sensors increases proportionally to the number of drives and the factor 2 or 3 with this arrangement of sensors. However, each additional current measuring sensor involves the danger of a potential source of errors and also causes additional costs which must be avoided.
On the other hand, in the variant of current measuring sensors in the common supply lines to several electrical machines, it is not possible to make more precise statements about the condition of the individual machines, since only a kind of average information of all machines is available. Although current measuring sensors can be saved thereby, this is done at the expense of the usable information.
In addition, in the case of current sensors being arranged in the same phases (e.g. JP 2002112404A) of several parallel machines, it is not possible to control them specifically or to determine the operating condition of individual machines, since only measured values are compared, but no sufficient information is obtained that might be used as the basis of an exact control.
In contrast, it is the object of the present disclosure to remove or reduce at least individual disadvantages of the conventional art. The present disclosure aims at providing a method or a device of the type mentioned above, which allows current sensors to be saved, without resulting in any greater loss of information.
According to the present disclosure it is provided that in two supply lines of the at least one first machine and in one of the supply lines of the at least one second machine, the currents are measured with the aid of current measuring sensors in the respective supply lines and, on the basis of the measured current values of the at least one first machine, the corresponding unmeasured currents of the at least one second machine are estimated, and in that the estimated total current of the supply lines of the at least one second machine is determined (or recorded) as a measure of deviation of the currents of the at least one second machine from the at least one first machine.
The method according to the present disclosure is therefore based on the assumption that the same currents flow in the unmeasured supply lines of the at least one second machine as in the corresponding supply lines of the at least one first machine. “Corresponding” in this context means “the same supply line/phase”. For small deviations of the load, this estimate will be accurate in good approximation.
Three winding strands or supply lines to an electrical machine may be designated U, V and W and added to the electrical quantities for differentiation as a subscript. In order to be able to distinguish the electrical values of the at least one first machine from the electrical values of the at least one second machine, index 1 or 2 is also used. Furthermore, estimated magnitudes are marked with a roof sign. The estimated current in the supply line U to the at least one second machine, for example, is therefore called ÎU2. Below, calculation is also performed in related quantities. If therefore the nominal current flows in a supply line, the current has the value 1.
The electrical machines used can be asynchronous machines. The currents in the supply lines of the at least one first machine may be determined accurately with only two current measuring sensors, since what applies is the relationshipIU1+IV1+IW1=0
In which two of the three supply lines of the at least one first machine the current measuring sensors are arranged is irrelevant due to the above relationship. Since the currents are known in the corresponding supply lines of the at least one first machine, one current measuring sensor can be inserted in each supply line of the at least one second machine without restricting the method. The two missing currents in the supply lines of the at least one second machine can be estimated at any time, since the corresponding currents of the at least one first machine are known.
By estimating the unmeasured currents in the supply lines of the at least one second machine, statements can be made about the condition of the at least one second electrical machine. Thus, when measuring the current in the supply line W of the at least one second machine, by forming the total currentÎU2+ÎV2+IW2=δ,
The measure of deviation can be determined, with ÎU2=IU1 and ÎV2=IV1 applying. The currents in the supply lines U and V of the at least one second machine are thus estimated by the currents in the supply lines U and V of the at least one first machine. Of course, the current measuring sensor can be arranged in any supply line of the at least one second machine. One advantage of the method disclosed is that the determined measure of deviation δ can be related to the measured currents, whereby a relative measure of deviation can be determined, which reflects the relative deviation between the at least one first electrical machine and the at least one second electrical machine. In addition, on the basis of the available information such as measured currents in the supply lines of the at least one first machine, a control system for the electrical machines can already be used.
If now the estimated currents correspond to the actual currents, i.e. ÎU2=IU2 and ÎV2=IV2, the measure of deviation is δ=0. This case usually occurs with the same load of the machines. However, if the estimated currents differ from the actual currents, then the measure of deviation δ is a positive or negative number, depending on whether the at least one second machine is loaded to a greater or lesser extent. In any case, the measure of deviation δ is no longer zero.
Furthermore, a first current space phasor may be determined from the measured currents in the supply lines of the at least one first machine and a second current space phasor is determined from the measured current in the supply lines of the at least one second machine and the estimate for the two currents that have not been measured. This makes it possible to use a suitable control based on a space phasor representation. The current space phasors may be calculated according to the following formula:i=⅔(IU+IV*ej120°+IW*ej240°).
The first and second current space phasors are designated i1 and i2 respectively. The first current space phasor i1 can be determined from measured or calculated values IU1, IV1 and IW1 to i1=⅔(IU1+IV1*ej120°+IW1*ej240°). The second current space phasor is calculated again to i2=⅔(ÎU2+ÎV2*ej120°+IW2*ej240°). Again, it was assumed that the one current measuring sensor is arranged in the supply line W of the at least one second machine, and the currents in the supply lines U and V are estimated. With the two current space phasors i1 and i2, various control methods known from the literature can be used.
In addition, it is possible that a torque τ of the at least one second machine is estimated on the basis of the second current space phasor. This is possible with known formulas for the torque of an electrical machine, for example τ==IM{Ψ i2*}. This requires other knowledge of the machine, such as the flux linkage space phasor. Their determination or calculation is also already known from the literature.
Furthermore, a torque of the at least one first machine can be calculated on the basis of the first current space phasor. Since the current space phasor i1 is calculated and not estimated, it is also possible to calculate the torque of the at least one first machine.
A measure of deviation of the currents of the at least one second machine from the at least one first machine is determined from the phase and/or magnitude difference between the first current space phasor and the second current space phasor and allows for better determination of deviations of the at least one first machine from the at least one second machine. If the estimated current values ÎU2 and ÎV2 correspond to the actual current values, the two current space phasors i1 and i2 have the same magnitude and the same phase. If the measure of deviation measure is δ≠0, there may be a difference in the magnitude and phase of the first current space phasor to the second current space phasor. The phase and/or magnitude differences therefore constitute an additional measure of deviation of the currents of the at least one second machine from the at least one first machine. For example, depending on the application, a threshold value could be defined, as of which a control takes a corrective action and counteracts a deviation of the two machines. Such a threshold value would be, for example, a 10-percent deviation of the magnitude of the two current space phasors i1 and i2. However, the specific value depends on the application. However, a threshold value based on the phase or, respectively, the magnitude and the phase would also be conceivable.
Furthermore, by applying voltage steps ΔU in the supply lines current changes occur, which current changes are detected by the current measuring sensors, and a winding resistance may be determined with the aid of the measured corresponding current changes. The voltage steps ΔU in the supply lines cause changed currents in the supply lines, which can be detected with the current measuring sensors. After transient effects (caused for example by inductances) have subsided, a current step ΔI can be determined. Therefore, the winding resistance may be calculated by quotient formation of the voltage step and the current step of a supply line according to Ohm's law. For the voltage and current jumps, the differential voltages and residual currents before and after the steps and after the decrease of transient effects are used.
Furthermore, the operating conditions of the machines can be determined, if the temperature in the windings may be calculated by using a temperature-dependent resistance model. For example, if a linear temperature model of the formR≈R0+(T−T0)*αis used, the temperature T in the windings can be determined if the resistance R0 at temperature T0, the temperature coefficient α and the determined resistance R are known.
To obtain additional information about the machine condition, an inductance can be determined by applying a voltage in the supply lines in which current measuring sensors are arranged and by measuring the current slope of the corresponding currents. For this purpose, voltage steps ΔU in the supply lines can be used again. If the current slope of a supply line is used immediately after the voltage step ΔU in the supply line, the inductance can be determined, neglecting the resistance. Depending on the position of the rotor, the inductance of a machine has different values due to slotting effects.
Furthermore, a change in the position of the rotor, such as a standstill, can be determined on the basis of the determined inductance. When the rotor does not rotate, the determined inductance remains the same. This means that a standstill can also be detected on the at least one second machine. If the same current increase is registered in several measurements on the at least one second machine in the measured supply line, it may be assumed that the rotor is stationary.
The device of the type mentioned above comprises current measuring sensors arranged in two of the first supply lines and in one of the further supply lines, wherein an estimating unit is connected to the current measuring sensors and designed for estimating the undetected currents in the further supply lines on the basis of the corresponding measured current values of the first supply lines. For example, the estimation unit may be available as an independent component in the form of a microprocessor or be integrated into other components.